Fix CA1825 warnings on empty array initialisation.

OpenRA.Mods.Common/Traits/SupportPowers/SupportPower.cs

@@ -9,6 +9,7 @@
  */
 #endregion
 
+using System;
 using System.Collections.Generic;
 using OpenRA.Traits;
 
@@ -45,7 +46,7 @@ namespace OpenRA.Mods.Common.Traits
 		[Desc("If set to true, the support power will be fully charged when it becomes available. " +
 			"Normal rules apply for subsequent charges.")]
 		public readonly bool StartFullyCharged = false;
-		public readonly string[] Prerequisites = { };
+		public readonly string[] Prerequisites = Array.Empty<string>();
 
 		public readonly string DetectedSound = null;